I have to admit, this has been incredibly eye-opening and has made me reflect on my bond with my stepchild, who is 6 (almost 7) years old by the way.
Whilst I may never be able to love SS like my own, I do agree that the love I feel may change over time, particularly when he becomes an adult; the bond we have is incredibly special and all your comments made me realise that. I've shed many tears about his situation with his mum and wanted to protect his pure heart as much as I could. It has been indoctrinated to SS that I am his STEPmum only and that he is never to call me mum, which is, in reality, factual but similarly 1) hurtful as the person who takes care of him every day, and 2) heartbreaking that he is not to make up his mind on who he deems true family himself. It has been very hard for a plethora of reasons and has taken a toll on me, perhaps causing me to harbour this unjust resentment towards SS - i.e., 'ruining' the family dynamic through no fault of his own. On the flip-side, I also have shed many happy tears of his continuous love for me, for example for once screaming my name out loud during an end-of-school-year performance out of sheer happiness to see me, and for giving me massages in the middle of labour contractions. He is a bundle of love and I am sure looking back on all of these memories will be more meaningful once he gets older.
I think there is indeed truth to what some of you pointed out regarding it being my first biological child and thus the overwhelming love a mum can experience in this time, causing (unfair although seemingly natural) comparisons. My biological son is only 9 months old which may be a massively contributing factor to my feelings. I also think considering a third child (second biological) may be crucial for long-term life-satisfaction.
I feel I may be in a baby bubble clouding my thoughts, and whilst the love I feel for both may or may never 'even out', I feel confident it will be a very special bond nonetheless. And the reminder of him being biologically related to my biological son is such a powerful reminder. In fact, one of our family members told us that his brother will never actually be his real brother, and we were absolutely fuming with her for saying such a thing. In SS's mind, that is his baby brother (no prefixes attached) and so that is the reality we live by.
I feel a lot more aware of my feelings now - their origins, for example - and will be able to rationalise them a lot more in the future. It is hard and I feel incredibly guilty, but I know I am where I am supposed to be, I am hopeful everything will work out, and I am fortunate he loves me so much.
I want to thank everyone for commenting, whether with opinions (even if not in favour of me), personal anecdotes, advice etc.